1. Field of the Invention
The present invention relates to an antenna failure detecting device, and more particularly to a device for detecting failure of an antenna which is disposed in the neighborhood of a signal operating circuit such as an amplifier or the like, commonly uses a transmission line of a transmission signal or reception signal or a transmission/ reception signal and a power supply line to the signal operating circuit and contains the signal operating circuit, wire breaking or short-circuit of a common used line, and failure such as breakaway or contact failure of a connecting portion such as a connector or the like when the connecting portion exists in the line concerned.
2. Description of Related Art
In general, an antenna of radio equipment is frequently provided at a place separated from the main body of the radio equipment. The position of the main body of the radio equipment is determined in consideration of resistance to environment, maintenance, operability or connectivity to other electronic equipment, however, the position of the antenna is usually determined by attaching importance to communication performance.
With respect to some radio equipment, a signal operating circuit such as a head amplifier, a pre-amplifier or an end-amplifier may be used to enhance the gain of the antenna. Normally, the signal operating circuit as described above is disposed in the neighborhood of the antenna element. For example, when the signal operating circuit is an amplifier, a more excellent signal-to-noise ration (S/N ratio) is achieved as the signal operating circuit is provided to be nearer to the antenna element.
In an antenna having a signal operating circuit installed therein, it is indispensable to supply power to the signal operating circuit. However, a ling for power supply is commoditized with a transmission line for a transmission signal or reception signal or a transmission/reception signal (hereinafter referred to as common line). By multiplexing DC power supply voltage with high-frequency (AC) transmission signal or reception signal or transmission/reception signal, the number of lines can be reduced and thus the cost can be reduced.
The antenna having the complicated construction as described above is lower in failure resistance than the antenna having a simple construction (only an antenna element and a signal line). This is because it has the signal operating circuit such as the amplifier or the like and thus the number of parts is increased. Furthermore, there is some probability that a trouble such as breaking or short-circuiting of the common line occurs. When a connecting portion such as a connector or the like exists in the common line, there is also some probability that a trouble such as breakaway or contact failure of the connecting portion occurs.
It is needless to say that such a trouble disturbs the operation of the radio equipment, and it is required to detect it and take a required countermeasure (notify or the like) by some method.
FIG. 6 is a conceptual diagram showing a related art (for example, see JP-A-2002-319907). In this figure, a radio equipment body 1 and an antenna 2 are connected to each other through a common line 4 in which a first resistor 3 is inserted. The common line 4 transmits the reception signal of the antenna 2 and the DC power supply voltage V occurring in a power source 5 of the radio equipment body 1 while multiplexing them. The power supply voltage V is applied to both the terminals of a signal operating circuit provided to the antenna 2, for example, a low noise frequency converter (LNB) 6 and a second resistor 7 connected to LNB 6 in parallel. “Z0” affixed to LNB 6 represents impedance of LNB 6, “Z1” affixed to the first resistor 7 represents the value (resistance value) of the first resistor 7, and “Z2” affixed to the second resistor 3 represents the value (resistance value) of the second resistor 3.
In the construction as described above, the voltage Vs across the first resistor 3 is proportional to the load current I flowing in the common line 4, and thus the trouble in the common line 4 or the antenna 2 can be detected on the basis of the across voltage Vs. For example, when wire breaking occurs at some midpoint of the common line 4 (the position of a mark X (P), conveniently), the load current I is equal to zero (the voltage Vs is also equal to zero). Furthermore, when LBN 6 is broken in wire or opened, the impedance of the antenna 2 is equal to Z1, and it is higher than normal one (a parallel value of Z0 and Z1), so that the load current I is reduced (the across voltage Vs is also lowered). Furthermore, when LBN 6 or the common line 4 is short-circuited, the load current I is maximum (the across voltage Vs is also maximum).
Accordingly, when any one state of “across voltage Vs=0”, “across voltage Vs<first reference voltage” and “across voltage Vs>second reference voltage” is set, occurrence of a trouble such as wire-breaking or short-circuiting of the common line 4, wire-breaking or opening of LBN 6 or the like can be detected.
However, the conventional antenna failure detecting device as described above has a problem that the line resistance of the common line 4 is large and thus the loss cannot be neglected. This is because the first resistor 3 is inserted in the common line 4 at all times, and the resistance value *Z2) of the first resistor 3 is added to the line resistance of the common line 4 itself. By reducing the value (Z2) of the first resistance 3, the loss can be reduced. However, if the resistance value (Z2) is reduced, the across voltage Vs of the first resistor 3 is also reduced, so that erroneous detection is liable to occur. Furthermore, a high-precision reference voltage (first and second reference voltages) adapted to the minute across voltage Vs must be used, and thus the cost-up is unavoidable.